The present invention relates to dispensers, in particular foam dispensers capable of producing a foam using a compressed gas.
Common foam and aerosol dispensers produce a foam or an aerosol spray using VOCs, where the VOCs are provided as a liquefied gas which acts as a propellant. For example, many aerosol dispensers use liquefied petroleum gas (LPG) or the like. However, environmental agencies in many different countries are currently attempting to phase out the use of VOCs in such dispensers due to the health risks associated with them, such as sensory irritation and respiratory problems. VOCs are also flammable and more expensive than compressed gas propellants.
Some existing foam dispensers produce foam by passing liquid and gas through small orifices, which results in the formation of bubbles via Rayleigh-Taylor instabilities at a discrete orifice. Due to this mechanism, the smallest size of bubble which can be produced by these small orifice foaming devices is approximately equal to the diameter of the orifice. Hence, to produce small bubbles, for example around 60 microns in diameter, it would be necessary for such small orifice foaming devices to include orifices having diameters of approximately 60 microns.
However, such small orifices can be both difficult and expensive to manufacture. In particular, in order to produce orifices having diameter of less than a millimeter in a material it is typically necessary to use specialized techniques such as laser drilling, which is expensive and not well suited to high volume/low cost manufacture. Also, laser drilling suffers from inherent limits on the aspect ratios of orifices that can be manufactured, where orifice length to width ratios are typically limited to 10 to 1. Therefore, in order to produce a very small orifice via laser drilling (for example of approximately 60 microns in diameter) then such orifices would need to be drilled into a thin material (of approximately 0.6 mm in thickness for an orifice diameter of 60 microns). This in turn places limitations on the materials that can be used
These small orifice foaming devices typically include a multitude of small orifices, as the use of only a single small orifice limits the rate at which gas can be incorporated into the foam. In foaming devices which use multiple small orifices, it is necessary to position these orifices at distances of separation equivalent to several orifice diameters in order to prevent the bubbles emerging at the orifices from re-agglomerating into larger bubbles. This requirement means that the small orifices cannot be provided using low cost materials such as fine meshes, sintered materials or porous materials because the orifices in these materials are not sufficiently separated. Therefore, manufacturers must rely on techniques such as laser drilling as explained above.
Also, drawing gas bubbles through a small orifice at a significant rate requires a significant pressure drop across the orifice. This can be created by passing a liquid past the orifice, but in the case of small orifices a high flow rate is required to create a sufficient pressure drop across the orifice. In turn, a significant pressure is required to drive the liquid at a sufficiently high flow rate. Additionally, the rate of gas entrainment into the liquid flow is highly dependent upon the liquid flow rate and the pressure on each side of the orifice, which can result in large variations in bubble sizes and gas phase volumes. For example, where small orifices are used in aerosol-type systems employing a compressed gas propellant the headspace pressure may vary between 0.5 bar and 8 bar, resulting in results in large variations in bubble sizes and gas phase volumes.
Finally, small orifices are very prone to blockages. For example, orifices having diameters of 60 microns can easily become blocked by dust, off-cuts of materials from manufacture, or components of a liquid formulation which can dry and set in the orifice.
To date, it has not been possible to produce satisfactorily high quality foams without the use of VOCs, while ensuring dispensing devices are suitably cost efficient to manufacture.